1. Field of the Invention
The present invention relates to a print head of solid state scan type or a print head comprising a linear line of light-emitting elements such as an LED print head.
2. Description of the Prior Art
A print head which is used most at present uses a laser diode and a rotating polygon mirror. A laser beam generated by the laser diode is reflected by the polygon mirror to scan a photoconductor in the main scan direction. However, such a print head has problems such as vibrations and noises due to the rotation of the polygon mirror. As a whole, a print head of solid state scan type or with light-emitting elements aligned linearly, such as an LED head, is more appropriate because of no mechanical scan and a small size thereof.
Previously, a print head of solid state scan type such as an LED head is used to control the turn on/off of laser beams of the LED elements in the unit of a bit according to a bi-level image data so as to print a bi-level image. In other words, because the print head of solid state scan type controls the turn on/off of beams in the unit of a bit, it cannot express a half-tone image.
Then, it is suggested to modulate a beam intensity (driving current) or to modulate a driving time (pulse width) in order to express a multi-level image with an LED print head (refer Japanese Patent laid open Publications 1-180577/1989 and 1-258965/1989). The modulation can express a multi-level image of image data up to about four bits while correcting the scattering of the characteristics of each light-emitting element. However, it is difficult to express a multi-level image of image data of eight bits or more due to the circuit area size and the cost.